1. Field of the Invention
The present invention relates to phase-change optical recording medium recordable and reproducible of information by optically changing a recording layer material by optical beam irradiation, and also rewritable, and a recording method thereof.
2. Description of the Related Art
As one of optical information recording media which is recordable, reproducible, and erasable of information by irradiation of laser beam, a so-called phase-change optical recording medium utilizing transition between crystal-amorphous phase or crystalxe2x80x94crystal phase is well known. Such recording medium has been utilized as recording medium for computers or image and sound related devices for its capability of overwriting with single beam and for its simple structure compared to an optical system of a drive unit.
The recording materials thereof include GeTe, GeTeSe, GeTeS, GeSeS, GeSeSb, GeAsSe, InTe, SeTe, SeAs, Gexe2x80x94Texe2x80x94(Sn, Au, Pd), GeTeSeSb, GeTeSb, Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te, and the like. Particularly, Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te has characteristic of high sensitivity and a clear profile of amorphous part, and is developed as a recording layer for mark edge recording.
In Japanese Patent Application Laid-Open No. 3-231889, for example, a recording layer represented by an Ixc2x7(III1xe2x88x92xcex3V xcex3)xc2x7VI type general formula wherein I is an element of the group I, III is an element of the group III, V is an element of the group V, and VI is an element of the group VI is disclosed. However, such a recording layer has a problem in repetitive recording property.
The recording layer used in information recording medium disclosed in Japanese Patent Application Laid-Open No. 4-191089 has a problem also in repetitive recording property although improvement in erasing ratio and high-speed recording can be attained.
The unrecorded part (crystallized part) of the recording layer used in information recording medium disclosed in Japanese Patent Application Laid-Open No. 4-232779 has a structure including a stable phase (AgSbTe2) and an amorphous phase present around the stable phase. Therefore, a minute grain boundary is present in the crystallized part, causing noises, although the repetitive recording property can be improved. This does not have a serious adverse effect on the recording property of optical recording medium having relatively low recording density such as CD-RW (Compact Disk-Rewritable) using a laser beam of recording and reproducing wavelength about 780 nm, but is an obstacle to the realization of high-density recording in DVD (Digital Versatile Disk)-RAM using a laser beam of 680 nm or less and having a recording density about 4 times CD-RW, further high-density DVD-RW, and the like. A problem is left also for repetitive recording property.
The structure of the crystallized part of the recording layer utilized in Japanese Patent Application Laid-Open No. 4-267192 is in the multi-phase state of AgSbTe2 phase separated from a uniform amorphous phase and the other phase (stable phase or amorphous phase). When the other phase above mentioned is the amorphous phase, it has the similar problem with the information recording medium in Japanese Patent Application Laid-Open No. 4-232779 described above, and when the other phase is the stable crystal phase, the problem arises in that the satisfying recording property cannot be obtained.
In Japanese Patent Application Laid-Open No. 1-277338, a composition expressed by the composition formula (SbxTe1xe2x88x92x) are disclosed as the recording material, wherein M is at least one element selected from Ag, Al, As, Au, Bi, Cu, Ga, Ge, In, Pb, Pt, Se, Si, Sn and Zn with composition range of 0.4xe2x89xa6x less than 0.7 and yxe2x89xa60.2. The structure of this composition range is based on Sb2Te3. The recording layer of this structure has a problem in repetitive recording property in the region where y is 0.7 or more.
In Japanese Patent Application Laid-Open No. 1-303643, it is taught that a single xcex3-phase was obtained and satisfying repetition property was obtained for the information recording media disclosed therein. However, it does not refer to the crystalline structure of this xcex3-phase, and thus has a problem in realizing a recording media conformable to high linear velocity and high density in the future.
In Japanese Patent Application Laid-Open No. 2000-79761, a recording layer approximating SbTe eutectic composition is disclosed. The recording layer disclosed therein utilizes the phase separation of Sb phase and Sb2Te3 phase. Sb is determined in the range of 0.5 or more and 0.7 or less, thus it is not conformable to high-linear velocity recording.
Recording medium utilizing a reversible phase change between a crystalline phase and an amorphous phase has been internationally utilized in rewritable recording media (CD-RW, etc.). For use in CD-RWs, speeding up in recording has been required accompanying the wide use, and the high-speed recording became also indispensable for the phase-change recording medium.
In high-speed recording, or in recording at a high linear velocity, the reference clock of the emitting pulse of laser beam is shortened in accordance with the increase in recording density. Therefore, in order to form a mark (amorphous phase), both the emitting (heating) time of one optical pulse and the time of emitting a lower power necessary for cooling (or often emitting no power) are shortened, compared to the recording at a lower linear velocity.
Further, in an optical recording which utilizes LD (laser diode), since the rise time and the fall time of the LD are limited, it is difficult to form a mark by heating and cooling at one optical pulse. For this reason, the heating can be performed by emission of higher recording power, however the recording power has a limit in recording. Also a certain cooling time should be considered for rapid quenching to be carried out after heating, however, the cooling time cannot be assigned too long. It is thus difficult to control the heating and cooling within the reference clock to record a mark of a prescribed length at a higher linear velocity.
On the other hand, it is also required to optimize the recording material of the phase-change recording layer and the medium structure.
In order ensure the repetitive recording property in high-speed recording, the crystallization rate (or crystallization speed) must be increased to raise the erasing ratio, and it is necessary to optimize the recording layer material and the composition ratio of each constituting element.
When the crystallization rate (or crystallization speed) is too high, however, the amorphous phase is difficult to form. In this case, the medium structure may be arranged to form a quench structure, but the recording power is also required. In Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te series and Sbxe2x80x94Te eutectic series, although the crystallization rate can be increased by increasing Sb quantity, it makes difficult to form the amorphous phase, and also deteriorates the storage property under high-temperature environment of mark, if an increase in the quantity is too much. When the medium structure is arranged to form the quench structure, a problem of deficiency in sensitivity arises. The causes of the deterioration in repetitive recording property in the high-speed recording include deterioration of not only the recording layer but also the upper protection layer located between the recording layer and the reflection layer, or dispersion of the constituting elements in the recording film or cracking of the protection layer by thermal impact, due to repetition of heating under high temperature and cooling in a short time. To cope with the speed up, studying of recording layer material, protection layer material and reflection layer material are required. For the same reason, study of recording method is also necessary.
Known recording layer materials conformable to high-linear velocity recording include for example, the one disclosed in Japanese Patent Application Laid-Open No. 2000-313170. Namely, it is expressed by the formula [(SbxTe1xe2x88x92x)yGe1xe2x88x92y]zM1xe2x88x92z, wherein M is In and/or Ga with a linear velocity about 7 times CD linear velocity and, particularly, a recording layer material consisting of 4 elements of Ge, In, Sb and Te is described as preferable example. However, when In is excessively added, it may speed up the linear velocity although an increase in reproducing optical power accompanies deterioration in both reproducing and repetition properties. Hence, the linear velocity cannot be increased too much.
On the other hand, in a recording method accompanying high-density and high-linear velocity recording, means for solving a problem caused by the rise time and the fall time became approximately equal to the heating pulse time is disclosed in Japanese Patent Application Laid-Open No. 9-134525. The technique disclosed herein relates to determination of a heating and cooling time of a single optical pulse to become equal to or more than the reference clock, and on the contrary, reducing the number of pulses to obtain a prescribed mark length, thereby improving the recording sensitivity and modulation factor.
These prior arts are mainly applied to rewritable CD, and even if applied to higher-density and higher-linear velocity recording, sufficient performances cannot be obtained.
A technique in terms of applying the above mentioned technique to high-linear velocity recording is disclosed in International Application Laid-Open WO 0072316. However, this technique also is not sufficient, and it is necessary to find out a technique having higher applicability and wider margin.
An object of the present invention is to provide a phase-change optical recording medium which performs a high-density recording at extensive linear velocities ranging from 3.5 m/s to a maximum of about 20 m/s, specifically 2 to 5-fold velocity (7.0-17.5 m/s) of the DVD, and further, to a phase-change optical recording medium easy in initial crystallization and excellent in repetition and storage properties, and a recording method thereof.